Force motor



Nov. 4, 1958 D. w, ERlCsQN 2,859,39

' FORCE MOTOR Filed June 7, 1955 2 Sheets-Sheet 1 A'rroRA/.srs

Fo'RcE Moron.

nousnrw; miesen; canarie, Mich. Appucsuon-Juns1,.19ss, serial-Nansen n'claime (Crist-#1m This invention relatesa to'anA improved constructionf for a force motor, which term' is employed to designateV a device in which electrical energy is directly converted to linear motion. v One4 example of the utility of a force motor is in positioning-of the spool of ahydraulic servo valve,- the-force motor being used as an element of an electronic controlling system and being adapted-by its operatingcharacteristicytoconvertan electrical signal' into adesired in this-. eldofuse.

As force motor essentially consists ofan armature I spool position'. The present inventionvwill be illustrated a mountedrfor linear movement in a frame structure which provides the. magnetic circuits requiredfor producing armature motion. For servo valve use, the type of armature motion` characteristics desired are such as to producea large force, withI small armature stroke, high natural` frequency, andwith the amount of motion bearing substantially astraight-line relationship to the amount of` co`nt`rol currentl A These desirable characteristics are achieved in the forcemotor of; the presentinventionby providing' a structure having two spaced, opposed pole pieces extending in the direction ofarmature movement, and movably supporting-A the armature between such pole pieces. ,A vquiescent iluxI of substantially constant density. is impressed uponthe armature from another pole piece, or preferably from a pair of pole pieces positioned laterally adjacent the armature and adjacent the gap between the first-mentioned .pair of p`ol e pieces. The magnetic circuit through `whiclrthequiescent ux flows-includes the amature, the fiugrdividing. equally between the pair of' poleY pieces which extend in' the direction of armature tn ovemerltv and returning through a magnetic patli of iricrerrient'alA reluctance and'- low total reluctance. Means are included for producing a-difference in fluxbetween the arr'nature and each ofthe pair of pole pieces it'iitlie direction o'fiarmature movement, and pref-. e'rblythis means cnsis'tsin a coil wound about each of the pair of pole pieces, and suitably connected to a-source of' cotiifeuifent'. .Tlie' magnetic circuit for this conti'diuxineluaes' slew reluctance rerum path between the raifqf'pblepieca:

United States Patent O ICC Z' from variation resulting. from changesin the amount of flux supplied by the controlv coils. i

The present invention alsocontemplatcs improved features' fo'r aforcelmotor assembly, including a spring system for.- suspending,l centering and opposing armature moverrient',. which permits all moving parts and electric circuits tobe completely sealed-against water, dirt, oil

' or any foreigny matter; and' a construction designed to facilitate the' use of aforcemotor of this-type in combination'with hydraulic control valves of the spooltype.

Otherobjects; features-and advantages of the invention will be more' fully described in connectionwitlr the explanation to be given ofthe principles of operation of the force motor and a presently' preferredconstructional ernbo'diment thereof particularly suitedfor .iydrauli.. servo valve useyallshown in the accompanying drawings which comprise thefollowing views;

Fig.: 1f, aschematic sectional elevation oi?l thearmature and magnetic structure taken along the lonftudinal renter linecfa-force motor incorporatngthe invention',

Figt22, a'schematicsectional elevation of the structure of Fig.- 1- taken at- 90 to the plane thereof. in order to completely disclose the magnetic structure;

FigA 3, as sectionalelevation of a representativel con'- structionalembodiment-'of theinvention, showing a force motor assembly arranged for usein conjunction with-the spool of. a hydraulic servo valve;-

Fig-. 4,- a' side elevation' of the force motor assembly portion'tofl the structure of- Fig. 3, taken on a plane 90 to the plane` of Fig.- 3; and' Figo 5; a, transverse section throughthecenter of the force motor assembly takenalong the line 5 5 of Fig.= 4.

Referringiirst to theschematic view s; 1 and2, which together show the complete relationshipv between an armature and the field vstructure inl which it is mounted, the motorl armatureisindicatedtby the reference 10, and consists of. a'parallelepiped block of material-havinghigii permeability. armature 10' is mounted on a shaft 11. Asbestshown in Fig. l, shaft 1=1 extends through the center ofthe polepieces 14 and 15 formingpart of two E-section members 12 and 13, respectively. Members 12 and 13 are made of material having. high permeability, and their pole pieces 14 and 15 are separatedby a gap dimensioned to provide forthe width of thearmature 10 plus a clearance preferably two to three times greater than-the desired amount of armature movement. Control coils 16 and 17 are wound about each of the pole pieces 14y and 15'.

Referring to Fig. 2, which is a schematic section taken at ,910" to the plane of Fig. l', the magnetic structure surrounding the armature is completed by two permanent magnet' members 1'8 `and 15.9; having pole pieces 20 and 'ifiissairangenienr ofthearmature and magnetic' cir- 'quits-'enables highfiux densities to-be'oitained and there for'e a largearmature' force' to l'ie4 produced by' a small difference-'2inl` flux between-the' two-coils; and within small limits of aimature't'ravei,tlie amount of force ou thearmature is proportional'to the-amount of. control current. Tlie"iiiclu`sion of sprin'g'neans" arranged to normally urge th'eartiiatuie t'da'centeied position results in armature movement' b'ecdning substantially proportional tov the a entretiennent; 'i

Preferably therqueseent fluir isV supplied by tfieiuse of a permanent'magnetwhich enables' this tlux to be imposed on the'armature without the necessity of empteyingaleonstant exciting current. The. usel ofa magnet also@ maken the: quiescent aux value substantially Afree 21,. respectively, -which are lpositioned adjacent the side of the armature and' adjacent the gap between pole pieces I4 and 151 '1`-li`e magnetic poles of the members 18`and 19` are 'rient'edia's-shown in the drawing and the sieand ux density producedl by each member 18 and-19 is identical; Consequently, the effect of the magnetic circuit shown in Fig. 2 is t'o c'ause two equal magnetic fluxes to en ter the armature from its sides 22 and 23. This ux divide'sand leaves the armature through its ends 24'and Z5 as' indicated by the' arrows A and BL The total force i on the armature in the direction" of shaft movement is proportional' tb 4L-:Bz inthe direction' of th' larger of the fluxes A andBl Normally, this difference is zero. In order to obtain a controllable' flux difference t'o"pro' duce' a force onthe' armaturein either direction, one of the coils: er' 17 can.v be.' energized,- or alternatively the twofcoilsll and 1-7 subjected to a different amount of euergiirlg'current The' difference in the'iiuxes'- A and Bfwill be proportionalE to'itheditference in' energizing current'land' the arm'aturowillbe subjected to a force toward the pole piece 14 or 15, depending upon the nature of the flux difference.

A centering spring, not shown in Figs. 1 and 2, is incorporated to provide a restoring force on the armature which keeps it normally centered between the poe pieces 14 and 15 when no force is developed by a ux difference. The centering spring permits the armature to be displaced from a centered position in an amount proportional to the force applied to the armature or in other words proportional to the coil differential current, which differentialcurrent will be generally termed the control current.

The flux difference AL-B2 depends upon the displacement of the amature from a centered position as well asupon the control current, 'but the effect of armature displacement is similar to that of a negative spring. In' other words, as the armature is displaced from a centered position, the net result is to produce an additional de-centering force. This effect can be counteracted by employing a centering spring which has a spring constant greater than the equivalent negative spring constant of the armature magnetic circuit to always provide a restoring effect tending to return the armature to a centered position.

The linear characteristic of armature movement in proportion to control current is obtained in this construction "by the arrangement of magnetic circuits described. The quiescent ux necessary for good sensitivity is provided by the permanent magnets, whose high incremental reluctance tends to keep the quiescent flux constant independent of armature displacement, while at the same time preventing variation of the reluctance of the path of ux from the two control coils. At the same time, this quiescent flux is obtained from the permanent magnets without the expenditure of power. It should be lremembered that a relatively small amount of movement (in the order of .005 to .025 inch), is required for satisfactory operation of a control device of this type in most instances.

A representative construction of a force motor assembly, incorporating the magnetic structure'previously described and shown in Figs. 1 and 2, is illustrated in Figs. 3 to 5. Referring to Fig. 3, the armature 10 is shown mounted on a tubular shaft 11 which extends through a hole 27 formed in each of the two E-section field members 12 and 13 and out through the center `apertures in an end cap 28, which is positioned adjacent the field member 12, and in an end cap 29 positioned adjacent the field member 13. The armature and shaft are suspended in the holes 27 and apertures, and centered between the poles 14 and 15 by a sys tem of springs which include a pair of leaf springs 30, each positioned between the mating faces of the members 12 and 13 and having their ends connected to the armature by a plug 31, all as clearly. shown in Fig. 5. Mounted within the end cap ,28 is a spring 34, this spring being positioned against an annular surface 35 formed in the end cap and bearing against a retainer ring 36 which in turn abuts against the end surface of the field member 12. Spring 34 is formed with a central hole through which the shaft 11 extends, and motion of the armature is transmitted to the spring by a spacer tube 37 surrounding the shaft 11 between the armature and spring 34.

A bellows 38 ls employed to seal the shaft aperture in the end cap l28. It is soldered to theend cap 28 and to a retainer 39 which has a portion 40 extending around the shaft and contacting the outer surface of the spring 34. This retainer 39 is positioned on the shaft by an end cap 42.

A similar construction is provided for the right-hand end of the assembly as the parts are viewed in Fig. 3. A second spring 44 has its outer circumference clamped -between an annular surface 45 on the end cap 29 and .a retaining ring-46 which contacts the end of the field member 13. Shaft 11 passes through an aperture in gives a sealing action which is free from wear due toY surface of the spring 44. The foregoing parts of the spring and bellows system are adjusted in assembled relation by the position of a nut 54 threaded on the end of shaft 11. After assembly, sealing of the parts is completed by soldering the end cap 42 and nut 54 to the shaft and the adjacent retainers 39 and 51, respectively.

The entire assembly is enclosed within a cylindrical casing 56, with a sealing ring 57 being employed between the casing 56 and end cap 28 and a second sealing ring 58 being employed between the casing 56 and end cap 29.

Suitable sealed feed throughs 62 extend through the end cap 29`and are electrically connected -by means not shown to establish independent circuits to each of the control coils 16 and 17.

Fig. 4, a side elevation of the complete force Wmfor assembly, shows the manner in which the pel....... ,t magnet 4members 18 and 19 are secured by screws-66 tothe field members 12 and 13.

Fig. 3 also illustrates one manner in which a force motor assembly can be used. Included in the view is a phantom showing of one end 70 of a hydraulic servo valve of conventional type having a spool (not shown) whose position is to be governed as a function of the position of the force motor shaft. Valve 70 is formed with an adaptor barrel 71 having an internally threaded end 72. The force motor assembly is mounted within a housing 74 formed with an integral end ring 75 to which the force motor assembly is secured by screws 76 engaging the end cap 28.- An externally threaded portion 77 is formed on the housing 74 to engage the internally threaded end 72 of the adaptor barrel 71. Position of the force motor assembly within the adaptor barrel' and relative to the spool to be controlled is established as desired by adjusting the threaded engagement between the housing 74 and the adaptor 71. The housing 74 is locked in this position by a ring nut 79. A cap 80, fitted with a suitable electrical connector plug 81 is secured to the end of the housing by screws 82, with a suitable internal sealing ring 83 being employed between these parts.

This illustrates the manner in which the structural and operating characteristics of the force motor are particularly suited for the direct control of a hydraulic valve. The entire unit, particularly the force motor assembly is completely sealed, and the employment of the bellows-type seals at the end of the force motor shaft the abrasive action of moving parts. The force motor assembly is thus good for a. long period of reliable service.

High performance characteristics are obtainable from the construction due to the small total gap required for a given amount of maximum linear displacement of the armature, and also due to the fact that the armature construction permits a minimum amount of armature massfor handling a given flux requirement. The armature and its spring system thus have a high natural frequency, highly desirable in servo systems where a fast, accurate response is required.

Other advantages of the construction will be apparent to those skilled in the art as will be possible modifications and deviations from the specific construction disclosed herein by way of example. Such modifications, included within the scope of the following claims, are to be considered a part of the invention.

Iclaim:

1. A force motor assembly having a field structure and an armature, said eld structure including a pair of oppop'o'le piecesfbeingconnected by' permeable material in'a applying a quiescent' nia'g'netic' thii'gli said'A ana'- ture between saidthird pole piece and each of said pair of pole .pi'e'cesj means for 'pr'odllcinga difference in flux between said armature andfeacho'f said'pait of pole pieces whereby a force is applied to said armature in a direction tbward nef' said 'p'air o'f'pole piec''s, and resilient means .opposing siicli fo'rc' a'd't'n'clirg' tb rtiin saidf armature to acentered p'dstiri4 btwe'e'n said' pair of pole pieces whereby.' armature mbvnelit'is" pfpitiohal inv direction and magnitude ti sid foi-'cie'.

2`. A`fo'r`ce`nforligf claim 1 wherein said means for applying a quiescent magnetic llux comprises a permanent magnet whose structure includes said third pole piece, said magnet being oriented so that said third pole piece forms one magnetic pole thereof, the opposite magnetic pole forming part of a magnetic circuit with said pair of pole pieces.

3. A force motor having a eld structure including a pair of aligned poles separated by a gap, an armature positioned in said gap and supported for linear movement on la shaft extending through an aperture in each of said pair of pole pieces, a control coil surrounding each of said pair of pole pieces, said field structure providing a magnetic circuit which includes said pair of pole pieces and said armature, said field structure further including means for subjecting said armature to a quiescent magnetic flux from a pole piece positioned intermediate said gap and laterally adjacent said armature and forming part of a second magnetic circuit which includes said armature and said pair of pole pieces, and resilient means opposing motion of said armature' whereby such motion is proportional in direction and magnitude to a dilerence in flux produced by said control coils. A

4. A force motor as set forth in claim 3 wherein said means for subjecting said armature toga quiescent ux comprises a pair of permanent magnets each having a common magnetic pole positioned adjacent said gap and laterally adjacent said armature, and a pair of opposite magnetic poles cach connected to said field structure to provide a magnetic circuit which includes said armature and each of said pair of poles.

5. A force motor according to claim 3 wherein said resilient means opposing movement of said armature and shaft are arranged to support said armature and shaft for free movement relative to said field structure.

6. A force motor according to claim 5 wherein said resilient means include a spring carried by said field structure and secured to said armature.

7. A force motor assembly including a field structure having a pair of opposed poles separated by a gap, an armature mounted between said pair of poles on a shaft extending through said field structure and beyond each end thereof, and end plates secured to said eld structure at each end of said assembly, said shaft extending through' said end plates, means supporting said shaft and armature for linear movement, bellows means for establishing a seal between said shaft and each of said end plates, a casing surrounding said magnetic eld and engaging said end plates, and means for establishing a seal between said end plates and said casing.

8. A force motor construction according to claim 7 wherein said means for supporting said shaft comprises a pair of springs each carried by one of said end plates and engaging said shaft, and a pair of leaf springs carried by said field structure and engaging said armature.

9. A force motor assembly according to claim 7 wherein said assembly is mounted in a housing having means for 6 l's'sigcollneeted in' adjustable;l sealed. relaties Awilli a hy: dr'aulie' valve uint.

10. A1 forse meter structure' eeafprlslng a palref'elfd" members wai'sl' are' E-sifspsd in lagifuainal secties, sala mer'nb'ers` being positioned i'n tl'ce-t'o-faceabutting r'la'- tion" t'o provide a'. pair" of op'pos'e'd p'ol'es separated' by a' gap', a' central longitudinalb'ore' formed in' eacli of said" field members, a sliftextldingthrugl saidl bores', an: armature rasantes on said shaftA am' p'ss'i'tleasdA in said' members and providing' a p'a'ir' of' quiescent llilic" p'o'le's' located' stfepposlte sizlsss'f` said' armature, a control cililrbn'ld o'n each" of said pair of opposed poles',- resiiinmens supporting s'aid shaft' and normally urging' said amature te' a eeateredposlfen.between said pairef opp'os'edp'ole's,aA cap t` e'ach` e`n`d o'f's'aid' struct'ur', each cap abutting one'v of said 'el'd rileir'lbe'rs andha'ving a'n a'p'ei'ti'rethgl which' said' shaftu passes, a` ci'ylir'ltlrical'A c's'slagisl'rrblidlgsard lllffelae'gs are lll-e' sides` sf' said caps, means esfaullsliingi a" 'sealjbetweea' said' .sap-s; and said casing', n'd` rea'ns` foi-"establishing a'r s'ec'o'nd seal' between said caps and said shaft.`

`1l. A force motor structure according to claim l0 wherein said resilient means supporting said shaft comprises a spring carried by each of said caps, each spring supporting said shaft and being responsive to motion of said armature, and a pair of leaf springs clamped between abutting faces of said field members, said leaf springs each being connected to said armature.

' 12. A force -motor structure comprising a pair of field members which are E-shaped in longitudinal section, said members being positioned in face-to-face abutting relation to provide a pair of opposed poles separated by a gap, a central longitudinal bore formed in each of said field members, a shaft extending through said bores, an armature mounted on said shaft and positioned in said gap, a pair of permanent magnets secured to said field members and providing a pair of quiescent ux poles located at opposite sides of said armature, a control coil mounted on each of said pair of opposed poles, resilient means supporting said shaft and normally urging said armature to a centered position between said pair of opposed poles.

13. A force motor according to claim 1 wherein the magnetic characteristics of the material which forms the magnetic circuits between said pole pieces provide a low reluctance path in the magnetic circuit between said pair of pole pieces and a high reluctance path in the magnetic circuit with said third pole piece.

14. A force motor assembly having a field structure and an armature, said field structure including a pair of oppositely disposed pole pieces spaced by a gap, means movably mounting said armature in said gap, a third pole piece laterally adjacent the gap between said pair of pole pieces, said pole pieces being connected by permeable rnaterial in a magnetic circuit, means for applying a quiescent magnetic flux through said armature between said third pole piece and each of said pair of pole pieces, means for producing a difference in ux between said armature and each of said pair of pole pieces whereby a force is applied to said armature in a direction towa'rd one of said pair of pole pieces, and resilient means opposing such force and tending to return said armature to a centered position between said pair of pole pieces whereby armature movement is proportional in direction' and magnitude to said force.

15. A force motor assembly having a eld structure and an armature; said field structure including a pair of oppositely disposed pole pieces separated by a gap; means and independent of armature displacement in a second4 magnetic circuit which includes said amature, said pair of pole` pieces and said third pole piece; ,means for pro-v duci'ng a difference in ux'between said pair of pole pieces whereby a force is applied to said armature in proporg pole pieces whereby armature movement is proportional in direction and magnitude to said force.

16. A force motor comprising field structure which includes a pair of poles separated by a gap, a bore formed through said field structure and the said paix` lof poles thereof, a shaft extending through said bore, an armature mounted on said shaft andl positioned in said gap, at least one permanent magnet forming part of said field structure and providing a qui'cscentlux pole located laterally of said armature and shaft,'a control coil mounted on each of said pair of poles, and r'esilientmeans supporting said shaft and 'normally urging said armature to a centered position between said pair of poles.

17.` A lf orce motor comprising field structure which includes a pair of poles separated by a gap, an' armature,

means supporting said .armature in said gap for back and forthl movement between said pair of poles, at least one permament magnet forming part of said field structure providing a quiescent flux pole located laterally, ofmsaid armature and its path of movement, a control coil mounte'd'zo' each of said pair of poles, and resilient means opposing movement of said armature.

References Cited in the le of this patent UNTTED STATES PATENTS 

